Infection with the spirochete, Borrelia burgdorferi, causes a multi-system illness in humans known as Lyme disease. The disease is regarded as the most common vector-borne human infection in the United States, Europe and parts of Asia. The pathogen persists in nature through an intricate enzootic cycle that involves small rodents and ticks. Immature stages of Ixodes scapularis acquire the pathogen while engorging on the dermis of a B. burgdorferi-infected wild rodent. The bacteria migrate to the arthropod gut, colonize the tissue and persist throughout the intermolt period. During a subsequent feeding, spirochetes within the tick gut multiply, disseminate to the salivary glands, and are transmitted to the host. B. burgdorferi, therefore, survives in diverse host environments in nature. We hypothesized that induction of selected genes by spirochetes, in precise host tissues, and time, could be important in maintaining B. burgdorferi in nature. Our preliminary data identified two B. burgdorferi genes encoding hypothetical proteins, bb0365 and bb0028, which display insignificant expression in infected mice but are dramatically induced as spirochetes are ingested by feeding ticks. The goal of the current proposal is to characterize the function of two vector-induced B. burgdorferi lipoprotein gene-products - BB0365 and BB0028 - to support the life cycle of spirochetes. In the current proposal, we will further study the expression profiles of bb0365 and bb0028 covering entire mouse-tick-mouse infection cycle of B. burgdorferi to identify precise times and host locations where these genes are most dramatically expressed. We will generate isogenic mutants of B. burgdorferi, deficient in bb0365 or bb0028, and use these gene-specific mutants to study function of relevant gene-products to support natural infection cycle of B. burgdorferi. This information will help us to understand the adaptive strategies of the bacterium that persists in extremely diverse hosts and may contribute towards development of preventive measures to interrupt the spirochete life cycle. Our preliminary studies identified two lipoprotein molecules of Lyme disease pathogen, Borrelia burgdorferi that are selectively induced as the microbe enters the feeding Ixodes ticks from the infected murine host. We now propose to study the function of these vector-induced spirochete gene-products to support natural infection cycle of B. burgdorferi. This information will contribute to our understanding of the adaptive strategies of B. burgdorferi through complex enzootic cycle and may contribute to the development of preventive measures to interrupt the infection cycle of the Lyme disease-causing pathogen.